Sporadic events of stress cracking in polymeric based film strips found in finished products but not observed in a 15 minute in-process check or AQL testing has been an issue with some polymeric based film products. Stress cracking generally begins to appear in the film product 2 to 4 weeks after manufacturing and packaging. Data analysis on lots exhibiting cracking has led to the conclusion that the problem is probably caused by imparting stress to the strips during the manufacturing process.
One source of stress that is imparted to the film during manufacturing is in the drying process. During drying, the wet film product is applied or coated to a substrate, and is then dried on that substrate. In some drying processes, particularly those including high solvent content, up to 70% of the volume of the wet film solution is removed in the drying process. This shrinkage imparts stress to the film, which adheres to the substrate onto which it is cast and dried. Another source of stress involves the cutting process. Following the initial drying and shrinkage, typically the film product is cut into individual rolls or strips. The film undergoes a number of bending operations as it transits the tortuous path of a slitting operation.
Finally, during fabrication and packaging of individual film strips, the process imparts additional stress by multiple turns around rollers, stripping the film off the substrate and by the pull wheels used to move the film product through the converting/packaging machine. When the strip is individually packaged in a material with a high surface energy, the film sticks to the packaging material and is not allowed to move to relieve the stresses. The stresses are thus relieved at a later time by stress cracking.